1. Field of the Invention
The present invention relates to a process for the preparation of aldehydes having 7 to 25 carbon atoms by multistage cobalt- or rhodium-catalyzed hydroformylation of the corresponding olefins.
2. Description of the Background
As is known, higher aldehydes, in particular those having 7 to 25 carbon atoms, can be prepared by catalytic hydroformylation (referred to as the oxo process) of the olefins which have one fewer carbon atom. The aldehydes are used, for example, as precursors for the production of carboxylic acids and as fragrances. In industry they are often converted into the corresponding alcohols by catalytic hydrogenation, such alcohols being used inter alia as intermediates for the preparation of plasticizers and detergents.
A large number of processes for the hydroformylation of olefins is described in the literature. The choice of catalyst system and optimal reaction conditions for the hydroformylation are dependent on the reactivity of the olefin used. The effect of the structure of the olefin on its reactivity in the hydroformylation reaction is described, for example, by J. FALBE, New Syntheses with Carbon Monoxidexe2x80x9d, Springer Verlag, 1980, Berlin, Heidelberg, New York, pages 95 et seq.
As a general rule, the rate of hydroformylation reactions under constant general conditions decreases with increasing carbon number and with increasing degree of branching of the olefin. Thus, the reaction rate of linear olefins can exceed that of the branched isomers by more than a factor of ten. In addition, the position of the double bond in the olefin has a decisive influence on the reactivity. Olefins with a terminal double bond react markedly more quickly than isomers with the double bond inside the molecule. The varying reactivity of isomeric octenes has been investigated, for example, by B. L. Haymore, A. van Hasselt, R. Beck, Annals of the New York Acad. Sci., 1983, 415, 159-175. A general overview and further literature are given by B. Cornils, W. A. Herrmann, xe2x80x9cApplied Homogeneous Catalysis with Organometallic Compoundsxe2x80x9d, Vol. 1and2, VCH, Weinheim, New York, 1996.
Industrial olefin mixtures which are used as starting materials for the hydroformylation synthesis often contain olefin isomers of very different structures having differing degrees of branching and different double bond positions, and olefins of varying molecular weights. This is true in particular of olefin mixtures produced by di-, tri- or continuing oligomerization of olefins having 2 to 8 carbon atoms or other readily accessible higher olefins, or by cooligomerization of the olefins. Possible examples of typical olefin mixtures which are relevant industrially for the hydroformylation reaction are tri- and tetrapropene, and di-, tri- and tetrabutenes.
In the case of a hydroformylation reaction conducted on an industrial level, it is desired to achieve, in addition to a high conversion, a high selectivity in order to ensure optimal utilization of the raw material. To achieve a high conversion, in the case of olefins which react slowly, a relatively long reaction time and/or relatively high reaction temperatures must often be accepted. By contrast, more reactive olefins are converted to the aldehydes under the same reaction conditions in a much shorter time. If mixtures of olefins of varying reactivity are hydroformylated together, this leads to the need for relatively long reaction times in order to achieve adequate conversion also of the olefins which are more difficult to hydroformylate. However, the aldehydes produced from olefins which can be more readily converted are formed relatively quickly and are then present in the reactor alongside the olefins which are more difficult to hydroformylate. This leads to undesired secondary and consecutive reactions of the aldehydes, e.g. to hydrogenation, to condensation reactions and to the formation of acetals and hemiacetals. Primarily because of the varying reactivity of the olefin isomers, it is therefore difficult to achieve high conversions and also high selectivities during the hydroformylation reaction.
As well as the disadvantageous effect on the selectivity of the reaction, there are two other aspects of the hydroformylation reaction which mitigate against a joint hydroformylation of olefin mixtures in one stage to achieve high conversions. First, the relatively long reaction times for a pregiven capacity (or reactor performance) require relatively large reactor volumes. This is a distinct disadvantage, particularly since hydroformylation processes are processes which occur at increased pressure, and the investment costs for pressurized reactors increase exponentially with size. Secondly, control of the product properties of the aldehydes is limited, e.g. determined by the n/i ratio.
Processes for the two-stage hydroformylation of olefins are known. EP 562 451 and EP 0 646 563 describe the hydroformylation of mixtures comprising 1- and 2-butene where, in the first stage, the 1-butene is reacted in a heterogeneous reaction, i.e. in a multiphase system, optionally with the addition of a phase transfer reagent or solubility promoter and, in the second stage, a homogeneous, dissolved catalyst is used. According to EP 0 562 451, rhodium catalysts are used in both stages, while according to EP 0 646 563, rhodium catalysts are used in the first stage and cobalt catalysts are used in the second stage. According to EP 0 562 451, the olefin which is unreacted in the first stage, largely 2-butene, is hydroformylated in a second stage in a homogeneous phase and in the presence of rhodium as catalyst. In EP 0 646 563 this procedure is specified inasmuch as the unreacted olefin in the first stage leaves the reactor in gaseous form, together with carbon monoxide, hydrogen and butane produced by hydrogenation. This gas, optionally at compression, is passed to the second hydroformylation stage. The procedure of these two publications cannot be used with advantage for the hydroformylation of higher olefins, i.e. olefins having more than 5 carbon atoms, because the unreacted olefins can no longer be discharged in gaseous form from the first stage with viable expenditure because of their relatively high boiling points.
GB 1 387 657 describes a two-stage hydroformylation reaction in which the reaction product from the first stage is discharged in gaseous form and, after the aldehydes or alcohols have been removed by condensation, some of the off-gas from the first stage, which comprises unreacted olefins, is returned to the first stage, and the remainder is passed to the second reactor. This process concept is suitable for the hydroformylation of volatile olefins having no more than 5 carbon atoms, e.g. for ethylene or propylene. Like the processes mentioned above, this procedure is not advantageous for the reaction of higher olefins, since the vapor pressures of the olefins (and those of the aldehydes) are too low and the process therefore has to inevitably be conducted in the liquid phase.
WO 95/08525 describes a two-stage hydroformylation process in which the reaction mixture is discharged from the first stage as a gas. Allegedly, olefins having 2 to 20 carbon atoms, in particular 2 to 8 carbon atoms, can undergo reaction by the process. The hydroformylation is rhodium-catalyzed, and the catalyst is identical in both stages. The example describes the hydroformylation of propylene. As with the processes described above, higher olefins having more than 5 carbon atoms cannot be converted with advantage to hydroformylation product on an industrial scale because of the relatively high boiling points of the starting materials and products. Conversion in the gas phase is therefore energetically unfavorable.
A further variant of a two-stage hydroformylation process is described in DE 3 232 557. In the first stage, the olefins are hydroformylated using a cobalt catalyst and conversions of 50-90% are achieved, the cobalt catalyst is separated from the reaction mixture, and the aldehydes formed are introduced into a second hydroformylation stage together with the unreacted olefins. The ligand-modified cobalt catalyst used here affects not only further hydroformylation of the olefins, but also hydrogenation of the aldehydes to give the alcohols. In addition, the aldehydes produced in the first stage are exposed to the energetic reaction conditions of the second stage. This leads to consecutive reactions, in particular, to condensation reactions with the formation of high-boiling components.
Accordingly, one object of the invention is to provide a process for the preparation of higher oxo aldehydes or the corresponding alcohols from olefins or olefin mixtures which simultaneously achieves high conversions of olefin(s)with high selectivities to product.
Another object of the invention is to provide a process for the preparation of higher oxo aldehydes or the corresponding alcohols from olefins or olefin mixtures which is additionally distinguished by high space-time yields and offers more room for maneuver to control product properties.
Briefly, these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for hydroformylating olefins to give alcohols and/or aldehydes in at least one hydroformylation stage, each of which comprises:
a) hydroformylating olefins having a carbon atom content of 6 to 24 carbon atoms in the presence of a cobalt- or rhodium catalyst in a reactor to the point of conversion of olefin reactant to product of 20 to 98%;
b) removing the catalyst from the resulting liquid discharged from the reactor;
c) separating the resulting liquid hydroformylation mixture into a low-boiler fraction comprising olefins and paraffins, and a bottoms fraction comprising aldehydes and/or alcohols; and
d) reacting the olefins present in the low-boiler fraction in additional process stages comprising steps a, b and c and combining the bottoms fractions of process steps c) of all process stages.